1. Field of the Invention
The present invention relates to a microlens array sheet for a liquid crystal display, a method for attaching the same and a liquid crystal display equipped with the same.
2. Description of the Related Art
A microlens array sheet arranged with minute unit lenses such as convex lenses and concave lenses in plane is expected for applications to liquid crystal displays, optically coupled optical elements or image input apparatuses, and the research thereof has been developed.
The microlens array sheets are typically classified into two types. One of the types is a microlens array sheet made by forming and arranging controlled concave and convex units (minute unit lenses) on a base plate having a flat surface using a fine processing technology. The other is a microlens array sheet made by providing a distribution of refractive index to appropriate minute unit portions in a planar base plate, that is, so-called flat-plate microlens array sheet.
In liquid crystal displays, usually the display is performed by utilizing an electro-optical effect of liquid crystal molecules, that is, by using a liquid crystal cell arranged with optical shutters which change ray transmittance or reflectance by utilizing an optical anisotropy (anisotropy in refractive index), orientation, flowability and dielectric anisotropy of liquid crystal molecules, and applying an electric field or sending an electric current to appropriate display units. In the liquid crystal displays, there are a direct-sight type display in which an image displayed on a liquid crystal cell is directly observed, and a projection type display in which an image to be displayed is projected onto a screen from front side or back side and the projected image is observed.
The direct-sight type liquid crystal display (hereinafter, also referred to as merely "liquid crystal display" or "LCD") has a defect that the image quality changes depending upon direction of observation. Since generally it is set so that the best image quality can be obtained when observed from the normal direction of the display surface, the image quality deteriorates as the angle between the observation direction and the normal direction of the display surface becomes larger. If the angle becomes larger than a certain level of angle, the image quality is out of a range which an observer can accept. Namely, the liquid crystal display has a defect that the range of the angle of visibility which can obtain a good image quality (hereinafter, also referred to as merely "angle of visibility") is narrow.
The defect that the angle of visibility is narrow is particularly remarkable in a super twisted nematic mode, which is often applied to personal word processors or personal computers because of excellent properties thereof that the structure is simple, the productivity is excellent and a large capacity of display is possible, or in a twisted nematic mode displaying gray scales, which is used for, for example, television receivers. By this defect, when observed from a direction with an angle in the range of 10-50 degrees from the normal-line of the display surface (the angle is different depending on the case where the measuring direction for the angle is vertical direction or horizontal direction), the displayed image often cannot be recognized. Therefore, in practice, a plurality of persons cannot observe the display simultaneously, and it obstructs the application and development of the liquid crystal display.
As a method for enlarging the angle of visibility of the liquid crystal display by incorporating optical elements such as lenses onto the observation surface side for controlling the direction of ray transmission, there are a method wherein plano-concave lenses are disposed (JP-A-SHO 53-25399), a method wherein polyhedron lenses are disposed (JP-A-SHO 56-65175), a method wherein a transparent plate having prismatic projections is disposed (JP-A-SHO 61-148430) and a method wherein lenses are provided on respective display elements of a liquid crystal cell (JP-A-SHO 62-56930 and JP-A-HEI 2-108093), and further, besides these, there is a method wherein means for controlling a ray radiation direction of a back light source is added when it is a transmission type liquid crystal display (JP-A-SHO 58-169132, 60-202464 and 63-253329).
On the other hand, as a technology for solving a problem that, when a microlens array is combined with a liquid crystal display, it becomes difficult to observe the displayed image on account of a direct reflection on the surfaces of the lenses, proposed is a method wherein non-reflection coating layer such as an anti-reflection multi-layer thin film is provided on the surfaces of the lenses (JP-A-SHO 56-65175).
However, any of the conventional technologies as described above is poor in property for practical use and has not yet been able to solve the problem of angle of visibility. According to the investigation by the inventors of the present invention, the reason is in the fact that there are defects in the conventional technologies that obtained effect for enlarging the angle of visibility is poor and/or the image quality remarkably deteriorates.
Namely, in a method wherein a single concave lens is disposed, because a relatively large curvature is required for the lens, the thickness of the display becomes thicker in consideration of the thickness of the lens, and the feature of a liquid crystal display that it is thin is damaged as well as the displayed image becomes hard to be recognized because the observed image is contracted.
Further, in the so-called flat-plate microlens array sheet such as one wherein an area having a distribution of refractive index is provided in the interior of a planar base plate, for example, a glass plate, because a sufficiently large difference between refractive indexes cannot be obtained, the lens effect is insufficient, and a sufficient advantage for enlarging the angle of visibility cannot be achieved.
Furthermore, in a method wherein concave and convex surfaces of respective optical elements are exposed on a observation surface of a liquid crystal display, such as the conventional method for disposing plano-concave lenses, polyhedron lenses, lenticular lenses or prismatic projections of a transparent plate, not only the effect for enlarging the angle of visibility is poor, but also the display contrast ratio when the liquid crystal display is observed from the front side (from the normal-line direction of the observation surface), that is, the ratio of the luminance for displaying the brightest color to the luminance for displaying the darkest color, decreases.
In a state where there are rays entering from outside of a liquid crystal display, for example, rays from ordinary room lights (hereinafter, also referred to as merely "outer rays"), there is a case where a microlens array sheet reflects the outer rays while scattering-them and the whole of the image plane of the display becomes whity, thereby deteriorating the visibility, that is, the easiness for observing the display. Because this defect becomes more remarkable as the effect of the microlens array sheet for enlarging the angle of visibility is larger, it makes the technology for enlargement of angle of visibility more difficult.
Theoretically, the bad affection due to the reflection of outer rays can be reduced to a level to be ignored by increasing the luminance of a back light source of a liquid crystal display and radiating a quantity of light much larger than that of the outer rays reducing the display quality from the back side. In this case, however, because the output of the back light source is required to be larger, the features of small size, light weight, thinness and low power consumption in a liquid crystal display are lost and the use of the liquid crystal display equipped with such a back light source is considerably restricted.
Further, according to the investigation by the inventors of the present invention, even in the above-described method wherein a non-reflection coating layer is provided on lens surfaces, this defect cannot be solved. Because, since such a non-reflection coating layer operates effectively only to rays entering from a specified one direction, if the angle of observation is changed, the non-reflection effect due to the layer would decrease or disappear. Therefore, it is difficult to apply such a layer to a liquid crystal display requiring a property of observation from various angles.
Although there is a case where a so-called non-glare treatment (mat treatment) is applied for forming an anti-reflection coating layer in which fine random irregularity is provided on a surface, this method has only an effect for suppressing a mirror reflection and therefore any effect cannot be obtained even if it is applied to a lens.
Since the defect that the angle of visibility of a liquid crystal display is narrow is an essential problem on a liquid crystal display, the enlargement of the angle of visibility due to an improvement of an interior of a liquid crystal cell is limited, and a sufficient effect cannot be obtained by such an improvement.